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Is global warming to blame for record low Arctic sea ice?

Earth's icy skull cap, floating atop the Arctic Ocean, has reached its lowest summer extent since satellites first began keep in track in 1979, and by some estimates its lowest reach in nearly 1,500 years. As of Sept. 7, the Arctic Ocean's expanse of summer ice this month spanned less than 1.54 million square miles, nearly six times the size of Texas and some 45 percent less than for the average for the same month through the 1980s and '90s, according to the National Snow and Ice Data Center in Boulder, Colo.

And the ice is still retreating; the summer melt season typically ends in mid to late September.

The previous record low was set in 2007, a result of an unusual set of conditions – clear skies during most of the summer and wind patterns that drove large amounts of ice past Greenland and into the North Atlantic. This summer, no such "perfect storm" for ice loss appeared.

Instead, much of the ice left over from winter – coming out of a summer that until now had been the second lowest melt-back in the satellite record – was thin enough to break no matter which way the wind blew, according to NSIDC researchers.

Indeed, the ice hit hardest by the long-term decline is the thick ice that once survived several years of thaw and freeze. With more of the Arctic Ocean starting the freeze season as open water, an increasing proportion of winter ice heading into the melt season is relatively thin – more vulnerable to wind-driven break-up when the melt season returns, which can speed melting.

The summer sea-ice cover at the end of the melt season has been declining since the early 1970s, although since 1979 satellites have provided the most consistent measurements of the decline.

The decline coincides with warming at the top of the world that has been occurring twice as fast there as it has for the northern hemisphere as a whole as the global climate warms. Climate scientists attribute the general warming to rising concentrations of atmospheric greenhouse gases – mainly carbon dioxide – from burning fossil fuels since the start of the Industrial Revolution as well as from land-use changes.

The higher pace of Arctic warming, linked in part to rising greenhouse gases as well as to the interplay between ice, snow, and ocean that is reinforcing the warming trend, has implications for more than caribou and polar bears.

This so-called Arctic amplification increases the likelihood of severe weather at mid-latitudes in the northern hemisphere, where most people live, according to a study published earlier this year in the journal Geophysical Research Letters.

"The Arctic is warming so much faster than mid-latitudes, and it's that difference in temperature that drives the jet stream," a river of air that triggers and steers storms, says Jennifer Francis, an atmospheric scientist at Rutgers University who focuses much of her research on the Arctic and is the lead author on the study.

As the temperature difference shrinks, the jet stream's speed slows and the north-south meanders it makes as it snakes from west to east grow longer. Both changes slow the jet stream's pace, contributing to the blocking patterns that lead to persistent bouts of heat, cold, or precipitation.

In the fall, the heat the Arctic Ocean stores in summer is released as the air above it gets colder. This slows the return of sea ice. And it reduces the temperature contrast between Arctic and mid latitudes, Dr. Francis explains, contributing to the blocking patterns that can appear in the late fall and winter. The jet streams' elongated meanders can bring one storm after another to parts of the continent while keeping other parts relatively storm-free. And the slowdown in the jet stream's migration across the hemisphere sets up the blocking patterns that can hold those conditions in place for weeks.

In the spring and summer, a different process can reduce the temperature difference between latitudes, she continues, one she says that has received far less attention than declining summer sea ice.

"In the last few years, we've also had record-low snow amounts in June and July on land at high latitudes," she says, resulting from what she calls a very robust trend toward earlier spring weather that melts the snow.

The land dries out sooner, with less moisture available to evaporate and keep a rein on rising temperatures. Air over the land areas warms sooner, reducing the temperature contrast into the spring and summer.

The effect on temperatures from a landscape deprived of its normal supply of rain or snow has been operating on overdrive this year, bringing severe to exceptional drought to much of the US.

Francis adds that the blocking pattern that kept the center of the US virtually rainfall-free, held temperatures over Greenland high enough to trigger melting across the entire top of the ice sheet, and gave Britain a persistently dreary, rainy summer, is consistent with the effects she and her colleague, Stephen Vavrus, a climate scientist at the University of Wisconsin at Madison, identify in their study.